1. Field of the Invention
The present invention relates to interferometric sensors and, more particularly, the present invention relates to a method and apparatus for providing polarization-induced phase noise insensitive signal processing for interferometric sensors.
2. Description of the Related Art
When using interferometric sensors, the input light to the sensor is split into two paths (i.e., a reference path and a sensor path) and recombined. The reference path is a path from the transmitter to the receiver via a first path of the sensor, while the sensor path is the path from the transmitter to the receiver via a second path of the sensor. The path that experiences a length change due to a disturbance within the sensor, usually the longest path, forms the sensor path and the other path forms the reference path. The portions of fiber that are common to both the sensor path and the reference path define the lead fibers. The light beams that travel along the two paths are combined to form an interference signal that is altered by the magnitude of the disturbance. If the nominal path lengths are different, the interferometer is said to be unbalanced, and the imbalance is equal to the difference in time-delay experienced by the light propagating in the two paths. The change in length difference between the two paths is measured by extracting the phase of the interference between the light that has propagated the two paths. The visibility of the interference depends on the state of polarization (SOP) of the two interfering light beams. The SOP of the two interfering light beams depends on the input polarization state into the interferometer as well as the retardance and the orientation of the polarization eigenstates of the two paths of the interferometer. Although the SOP of the light propagating in the reference and sensor paths may begin parallel, the propagation along the fibers may alter the SOP of each light beam such that the SOPs of the two interfering light beams may no longer be parallel. As the SOP of the interfering light beams approach orthogonality, the visibility worsens, and if SOPs are orthogonal, the visibility is zero and the interference signal can not be measured. This effect is known as polarization fading. The interferometer has two polarization eigenstates that represent the maximum and minimum phase of the interferometer. Depending on the input SOP, the measured interferometer phase can be any value between the phases of the two polarization eigenstates. Thus, if the sensor is birefringent, fluctuations in the SOP of the lead fiber will induce phase noise.
In an application such as interferometric seismic sensor monitoring, the lead fiber from the interrogation unit to the sensor can be of substantial length and sensitive to environmental effects such as vibrations, bending and temperature. The noise performance of such sensor arrays may be limited by the polarization fluctuations in the lead fiber induced by environmental effects. See A. D. Kersey, M. J. Marrone, and A. Dandridge, “Observation Of Input-Polarization-Induced Phase Noise In Interferometric Fiber-Optic Sensors”, Optics Letters, 13(10):847–849, 1988.
Several methods have been proposed to eliminate the problem of polarization fading in interferometric sensors while there are few methods that eliminate the phase noise that is induced by variations in the input polarization to the sensor and the retardance variations in the sensor. The polarization-induced phase noise can be eliminated using depolarized light; however this method does not solve the fading problem. See A. D. Kersey, M. J. Marrone, and A. Dandridge, “Analysis Of Input-Polarization Induced Phase Noise In Interferometric Fiber-Optic Sensors And Its Reduction Using Polarization Scrambling”, IEEE Journal of Lightwave Technology, 8(6):338–845, 1990.
Several methods for reduction or elimination of the polarization fading problem are known. One known method uses Faraday rotating mirrors, as disclosed by A. D. Kersey et al. in “Polarisation Insensitive Fibre Optic Michelson Interferometer”, EI. Lett., Vol. 27, pp 518–19, 1991. This method allows for a simple source and detection system, but it works only for a Michelson interferometer configuration. Furthermore, the Faraday rotating mirrors may be expensive, space consuming, and sensitive to extreme thermal, electromagnetic and other environmental conditions.
Another widely used method is to use a polarization diversity receiver based on three polarizers that are angularly spaced by 120′, and the output with best visibility is selected. See N. J. Frigo et. al in “Technique For Elimination Of Polarization Fading In Interferometers”, E I. Lett. Vol 20, pp. 319–320, 1984.
Other known methods are based on active polarization control at the input to optimize the visibility of the interference, as disclosed by A. D. Kersey et. al. in “Optimization And Stabilization Of Visibility In Interferometric Fiber-Optic Sensors Using Input-Polarization Control”, J. of Lightwave Technol., Vol. 6, pp. 1599–1609, 1988. When several sensors are multiplexed, this method requires input-polarization control of each multiplexed sensor, which makes it impractical for remote and inaccessible sensor arrays. Alternatively, one can optimize the visibility of the worst sensor in the array. See M. Tur et. al. in “Polarization-Induced Fading In Fiber-Optic Sensor Arrays”, J. of Lightwave Technol., Vol. 13, pp. 1269–1276, 1995. A statistical treatment shows, the probability that the visibility is larger than 0.6 for all sensors in a 10-element sensor array is 80%, however the visibility worsens as the number of sensors is increased. The visibility can also be optimized by the use of the polarizer combined with active polarization control at the output end, as disclosed by K. H. Wanser et. al. in “Remote Polarization Control For Fiber-Optic Interferometers”, Opt. Lett., Vol. 12, pp. 217–19, 1987. In both cases the polarization controller is continuously adjusted to optimize the fringe visibility. These techniques require relatively complex systems to provide feedback signals to the polarization controller. The polarization modulator used for the polarization control must be capable of modulating the SOP in three dimensions on the Poincare sphere.
The only previously reported method that eliminates polarization induced fading and noise, is based on modulation of the SOP between two polarizations with a modulation frequency that is an odd multiple of one forth of the free spectral range (FSR) of the sensor, and detection of four independent interference signals. See E. Rønnekleiv in “Elimination Of Polarization Fading”, International patent application number WO 00/79335 (filed Jun. 22, 2000). In systems that employ a continuous wave source such as wavelength division multiplexing (WDM), the minimum modulation frequency of one forth of the sensor FSR, gives a minimum detection bandwidth equal to the sensor FSR. In conventional CW interrogation, the minimum detection bandwidth is given by the information bandwidth of the interferometric signal. Thus, the minimum detection bandwidth required for this method is much larger than necessary for CW interrogation of interferometric sensors. In time division multiplexing (TDM) two-pulse interrogation, as disclosed in J. P. Darkin in “An Optical Sensing System”, UK patent application number 2126820A (filed Jul. 17, 1982), the four independent interference signals must appear within one sensor imbalance. Thus, the source polarization must be modulated with a modulation frequency that is at least 5/4 of the sensor FSR, which is the inverse of the sensor imbalance. The duration of the detected pulses is at maximum ⅕ of the duration of the detected pulses with conventional two-pulse interrogation, and thus the detection bandwidth is at least five times higher. For a typical sensor imbalance of 5 m, the FSR is equal to 20 MHz, and the required detection bandwidth must be at least 100 MHz. This high detection bandwidth makes this method impractical for TDM two-pulse interrogation.
Therefore, there is a need in the art for a method and apparatus that eliminates the polarization-induced signal fading and provides polarization-induced phase noise insensitive signal processing for interferometric sensors.